Freeze-fracture and deep-etch studies of chloroplast membranes have led to the development of a membrane model containing five basic types of particles (globular complexes of proteins) associated in different ways with the lipid bilayer membrane continuum. Two of these particles (coupling factor 1 and carboxydismutase) that are associated with the external membrane surface have already been positively identified. In contrast, the function of the three remaining particles (all integral type membrane proteins) has yet to be determined. Several lines of circumstantial evidence, including their nonrandom distribution between stacked and unstacked membrane regions, suggest that these intramembranous particles may correspond to complexes of photosysytem I (PSI) and photosystem II (PSII) components. The central goal of this proposal is to positively identify the function of these particles. To this end the following experimental approaches will be used. a) Labeling of chloroplast membranes with antibodies prepared against purified PSI and PSII complexes. b) Structural observations on reconstituted membranes prepared from purified PSI and PSII complexes and lipids. c) Chloroplast greening experiments in which the appearance of PSI and PSII activities will be correlated with the appearance of membrane particles. d) Membrane solubilization experiments using digitonin, and e) Chloroplast senescence experiments in which the disappearance of biochemical activities will be correlated with changes in membrane structure.